1. Field of the Invention
This invention relates to transistors and more particularly to transistors and methods for making ohmic contact to a transistor incorporating selective area growth.
2. Description of the Related Art
Improvements in the manufacturing of AlGaN/GaN semiconductor materials have helped advance the development of AlGaN/GaN transistors, such as high electron mobility transistors (HEMTs) for high frequency, high temperature and high power applications. AlGaN/GaN has large bandgaps, high peak and saturation electron velocity values [B. Gelmont, K. Kim and M. Shur, Monte Carlo Simulation of Electron Transport in Gallium Nitride, J. Appl. Phys. 74, (1993), pp. 1818-1821]. AlGaN/GaN HEMTs can also have 2DEG sheet densities in excess of 1013 cm-2 and relatively high electron mobility (up to 2019 cm2/Vs) [R. Gaska, et al., Electron Transport in AlGaN—GaN Heterostructures Grown on 6H—SiC Substrates, Appl. Phys. Lett. 72, (1998), pp. 707-709]. These characteristics allow AlGaN/GaN HEMTs to provide very high voltage and high power operation at RF, microwave and millimeter wave frequencies.
U.S. Pat. No. 5,192,987 to Khan et al. discloses GaN/AlGaN based HEMTs grown on a buffer and a substrate. Other AlGaN/GaN HEMTs and field effect transistors (FETs) have been described by Gaska et al., High-Temperature Performance of AlGaN/GaN HFET's on SiC Substrates, IEEE Electron Device Letters, 18, (1997), pp. 492-494; and Wu et al. “High Al-content AlGaN/GaN HEMTs With Very High Performance”, IEDM-1999 Digest, pp. 925-927, Washington D.C., December 1999. Some of these devices have shown a gain-bandwidth product (fT) as high as 100 gigahertz (Lu et al. “AlGaN/GaN HEMTs on SiC With Over 100 GHz ft and Low Microwave Noise”, IEEE Transactions on Electron Devices, Vol. 48, No. 3, March 2001, pp. 581-585) and high power densities up to 10 W/mm at X-band (Wu et al., “Bias-dependent Performance of High-Power AlGaN/GaN HEMTs”, IEDM-2001, Washington D.C., Dec. 2-6, 2001).
Field plates have been used to enhance the performance of GaN-based HEMTs [See S Kamalkar and U. K. Mishra, Very High Voltage AlGaN/GaN High Electron Mobility Transistors Using a Field Plate Deposited on a Stepped Insulator, Solid State Electronics 45, (2001), pp. 1645-1662]. Recently, field plate optimization for operation at microwave frequencies has resulted in drastically improved power densities exceeding 30 W/mm at 4 and 8 GHz [Wu et al, 30 W/mm GaN HEMTs by field plate optimization, IEEE Electron Device Letters, Vol. 25, No. 3, March 2004]. However, the reliability of these devices is still an issue, especially at high operation temperatures.
In present technology, ohmic contacts to GaN HEMTs are typically formed by alloying metal contacts at high temperature. This process results in ohmic contacts with rough morphology, which is undesirable for reliable devices and robust manufacturing. Furthermore, it has been proposed that alloying causes metal spiking into the semiconductor reducing the breakdown voltage of the device.
A typical prior art process for forming ohmic contacts on GaN HEMTs consists of depositing Ti/Al/Ni/Au or other similar metals on the semiconductor surface and then alloying the metals at a high temperature (>800° C.). This process results in ohmic contacts with rough morphology and a reduction in device breakdown voltage due to the spiking of the ohmic metal into the semiconductor.
Some technologies for non-annealed or low-temperature annealed ohmic contacts exist. These technologies include Si implants under the ohmic contacts, n+ GaN caps on top of the AlGaN/GaN structure, and selective area growth of n+ GaN in the contact region. These technologies, however, add complications to the process. In some of these prior art processes the regrowth mask is removed after the regrowth is performed. The mask removal can be difficult, leaving mask residue on the semiconductor surface. This problem can be exacerbated by a change in density or recrystallization of the regrowth mask during the high temperature regrowth stage.